Carbon nanotubes (CNTs) are allotropes of carbon with a unique atomic structure consisting of covalently bonded carbon atoms arranged in long cylinders with typical diameters in the range of 1 to 50 nm and a wide variety of lengths (Rubber Nanocomposites: Preparation, Properties and Applications; edited by Sabu Thomas and Ranimol Stephen, John Wiley & Sons, 2010). Based on the fast growing knowledge about the physical and chemical properties, nanosize carbon structures such as carbon nanotubes or carbon nanofibers (CNT or CNF) have found a wide range of industrial applications including field effect transistors, one-dimensional quantum wires, field emitters and hydrogen storage. Individual carbon nanotubes are characterized by a high aspect ratio (300 to 1000), high flexibility and unique combination of mechanical, electrical and thermal properties. The combination of these properties with a very low mass density makes them potentially useful as ideal reinforcing fibers for high-performance polymer composites.
However, one of the main problems for the effective use of carbon nanotubes as reinforcements of polymer matrices is achieving a good dispersion in the composite, independent of filler shape and aspect ratio. Unless uniform dispersion of CNT within the polymer matrix is obtained, enhancement in mechanical strength and other relevant physical properties is not achieved. Direct incorporation of CNT into dry natural rubber through mixing processes like those used for other common fillers is not as easy as, for example, the incorporation of carbon black. Rubber is a very viscous material. It is a very difficult task to disperse a very light material such as CNT into a very viscous medium such as natural rubber and other elastomers. Conventional mixing equipment, such as 2-roll mills, kneaders and internal mixers or even twin screw extruders, is not able to provide efficient dispersion of CNT in the rubber matrix.
Most reports and publications concerning nanoparticulate fillers for polymers relate to thermoplastics, but almost none to dry rubber. The main reason is that it is more difficult to mix nanoparticulate fillers into rubber than into thermoplastics since the former is a much more viscous material than the latter because the molecular weight of rubber is substantially higher than that of thermoplastics. The most important aspect of mixing is the final dispersion of the filler in the rubber matrix.
Carbon nanotubes as usually supplied consist largely of aggregates, but reinforcement comes from individual particles. Intercalation and exfoliation denote CNT dispersion and interaction with the polymer matrix, respectively. If intercalation and exfoliation are not attained during mixing, the final outcome is very poor mechanical strength. Thus, mixing of CNTs with rubber using conventional methods does not produce the desired physical properties and mechanical strength. The root cause of the problem is associated with the poor dispersion of nanocarbon in the rubber matrix due to the high viscosity of dry rubber.
Chinese patent application CN 1663991 A describes a powder natural rubber modified by CNTs and a method of preparing the same. Said powder natural rubber is characterized in that the mass ratio of CNTs to dried rubber of natural rubber latex is in the range from 1% to 50%. The method of preparing the modified rubber requires that the CNTs are subjected to an acid treatment to make them hydrophilic. The method further comprises the steps of mixing the treated CNTs with a dispersant and deionized water to form a CNT/water slurry; modifying the pH value of the slurry to 9 to 12; mixing the slurry with natural rubber latex to form a natural rubber liquid latex added with CNTs; and spraydrying the latex to obtain the powder natural rubber modified with CNTs.
Similarly, Chinese patent application CN 1673261 A describes a natural rubber liquid slurry added with carbon nanotubes characterized in that the total solid contents of CNTs and the dried rubber of the natural rubber latex is in the range from 5% to 30% and a method of preparation such a natural rubber liquid slurry, characterized in that the method comprises the steps of (i) surface treating CNTs such that they become hydrophilic; (ii) mixing the CNTs with dispersant and deionized water to obtain a CNT/water suspension, wherein the mass ratio of dispersant to the said CNTs is in the range from 5% to 20%; (iii) adjusting the pH of the suspension to 9 to 12; and (iv) homogenously mixing the pH adjusted CNT/water suspension with natural latex to obtain a natural rubber liquid slurry with added CNTs.
However, there remains a need to simplify the preparation of nanocarbon reinforced natural rubber and to improve the mechanical properties thereof.
Thus, it is an object of the present invention to provide a simple way to overcome the problem of the very high viscosity of dry natural rubber which consequently leads to agglomeration and very poor dispersion of nanocarbon in the rubber matrix which in turn results in poor mechanical properties, especially poor mechanical strength. It is a further object of the present invention to provide a natural rubber composition reinforced with nanocarbon that has or results in improved physical and mechanical properties, such as improved hardness, improved modulus and/or improved tensile strength.